1. Field of the Invention
The present invention relates to an exhaust purification system for an internal combustion engine, and particularly relates to an exhaust purification system for an internal combustion engine equipped with a selective reduction catalyst that reduces NOx in the exhaust under the presence of a reducing agent.
2. Related Art
Conventionally, as one exhaust purification system that purifies NOx in exhaust, a system has been proposed in which a selective reduction catalyst that selectively reduces NOx in the exhaust by way of a reducing agent is provided in an exhaust channel. For example, with an exhaust purification system of urea addition type, urea water is supplied from an upstream side of the selective reduction catalyst, ammonia is generated by thermal decomposition or hydrolysis of this urea water by the heat of the exhaust, and the NOx in the exhaust is selectively reduced by this ammonia. In addition to such a system of urea addition type, for example, a system has also been proposed that generates ammonia by heating a compound of ammonia such as ammonia carbide, and directly adds this ammonia. A system of urea addition type will be explained hereinafter.
With such a selective reduction catalyst, in a case of the injection amount of urea water being less than an optimum amount, the NOx purification rate declines from the ammonia being consumed in the reduction of NOx being insufficient, and in a case of being larger than this optimum amount, the ammonia that has become surplus in the reduction of NOx is discharged. As a result, appropriately controlling the injection amount of urea water has been important in exhaust purification systems provided with a selective reduction catalyst. Therefore, in Patent Document 1 and Patent Document 2, systems are exemplified that estimate a NOx purification rate of the selective reduction catalyst, and control an injection amount of urea water based on this estimation.
With the exhaust purification system disclosed in Japanese Unexamined Patent Application Publication No. 2004-100700 (hereinafter referred to as Patent Document 1), the NOx amount on a downstream side of the selective reduction catalyst is detected by a NOx sensor, and the composition of the exhaust flowing into the selective reduction catalyst, more specifically, the ratio of NO and NO2, is estimated from the output value of this NOx sensor and the operating state of the internal combustion engine. Furthermore, based on this composition of the exhaust, the NOx purification rate of the selective reduction catalyst is estimated and the injection amount of urea water is controlled.
In addition, with the exhaust purification system disclosed in Japanese Unexamined Patent Application Publication No. 2006-274986 (hereinafter referred to as Patent Document 2), the temperature of the catalyst is detected as an amount relating to the NOx purification rate of the selective reduction catalyst, and the injection amount of urea water is controlled based on this temperature.
However, the NOx reduction rate of the selective reduction catalyst changes not only by the above such composition of the exhaust and temperature of the selective reduction catalyst, but also according to the degradation state of the selective reduction catalyst. In addition, there is variability in purification performance between individual units. Therefore, it is difficult to always optimally control the injection amount of urea water with exhaust purification systems such as those exemplified in Patent Documents 1 and 2.
Consequently, a technique is considered below that more directly detects the NOx purification rate of the selective reduction catalyst, and controls the injection amount of urea water based on this.
FIG. 19 is a schematic diagram showing a configuration of a conventional exhaust purification system 80.
As shown in FIG. 19, an oxidation catalyst 83, a urea injection valve 85 that injects urea water as a reducing agent that is stored in a urea tank 84 into an exhaust channel 82, and a selective reduction catalyst 86 that reduces NOx in the exhaust under the presence of ammonia are provided in sequence from an upstream side to a downstream side in the exhaust channel 82 of an engine 81. In addition, a temperature sensor 87 that detects a temperature of the selective reduction catalyst 86 and a NOx sensor 88 that detects a NOx amount on a downstream side of the selective reduction catalyst 86 are provided as sensors for observing the purification performance of the selective reduction catalyst.
With this exhaust purification system 80, for example, the NOx amount of exhaust discharged from the engine 81 is estimated by way of a map set in advance, and the injection amount of urea water from the urea injection valve 85 is determined based on this NOx amount and the catalyst temperature detected by the temperature sensor 87. In particular, herein, the degradation state of the selective reduction catalyst 86 can be estimated based on a difference between an output value of the NOx sensor 88 and the NOx amount of exhaust thus estimated. With this exhaust purification system, the injection amount of urea water is corrected in accordance with the degradation state of the selective reduction catalyst 86 estimated in the above way.